Cementing operations are used in wellbores to fill the annular space between casing and the formation with cement. When this is done, the cement sets the casing in the wellbore and helps isolate production zones at different depths within the wellbore from one another. Currently, the cement use during the operation can flow into the annulus from the bottom of the casing (e.g., cementing the long way) or from the top of the casing (e.g., reverse cementing).
Due to weak earth formations or long strings of casing, cementing from the top or bottom of the casing may be undesirable or ineffective. For example, when circulating cement into the annulus from the bottom of the casing, problems may be encountered because a weak earth formation will not support the cement as the cement on the outside of the annulus rises. As a result, the cement may flow into the formation rather than up the casing annulus. When cementing from the top of the casing, it is often difficult to ensure the entire annulus is cemented.
For these reasons, staged cementing operations can be performed in which different sections or stages of the wellbore's annulus are filled with cement. To do such staged operations, various stage tools can be disposed on the casing string for circulating cement slurry pumped down the casing string into the wellbore annulus at particular locations.
For example, FIG. 1A illustrates an assembly according to the prior art having a stage tool 24 and a packer 22 on a casing string 20, liner, or the like disposed in a wellbore 10. The stage tool 24 allows the casing string 20 to be cemented in the wellbore 10 using two or more stages. In this way, the stage tool 24 and staged cementation operations can be used for zones in the wellbore 10 experiencing lost circulation, water pressure, low formation pressure, and high-pressure gas.
As shown, an annulus casing packer 22 can be run in conjunction with the stage tool 24 to assist cementing of the casing string 20 in the two or more stages. The stage tool 24 is typically run above the packer 22, allowing the lower zones of the wellbore 10 to remain uncemented and to prevent cement from falling downhole. One type of suitable packer 22 is Weatherford's BULLDOG ACP™ annulus casing packer. (ACP is registered trademarks of Weatherford/Lamb, Inc.)
Other than in a vertical bore, stage tools can be used in other implementations. For example, FIG. 1B illustrates a casing string 20 having a stage tool 24 and a packer 20 disposed in a deviated wellbore. As also shown, the assembly can have a slotted screen below the packer 22.